There is a great deal of interest at the present time in optical logic elements not only because of their potential capability for performing fast logic operations but also because of the possibility they afford for construction of massively parallel computer architectures. It is contemplated that a single array of optical logic elements might contain at least 10.sup.6 logic gates which would function simultaneously. Several such arrays might be optically interconnected by a series of lenses thus permitting operation of more logic elements in a given time period than is presently contemplate for electronic logic elements.
Several types of optical logic elements have been developed. For example, highly nonlinear semiconductor materials such as InSb, InAs or GaAs may be used in optical bistable devices. See, for example, Applied Physics Letters, 42, pp. 131-133, Jan. 15, 1983. Use of such semiconductors in multiple quantum well (MQW) devices relying on absorption effects caused by excitons has also been demonstrated. One promising MQW device is termed the self-electro-optic effect device (SEED) and uses optically influenced electric fields to modulate the light beam. See, for example, Applied Physics Letters, 45, pp. 13-15, 1984. These elements may be termed single beam logic elements.
Yet another approach to optical logic elements uses a nonlinear Fabry-Perot etalon to form logic gates. See, for example, Applied Physics Letters, 44, pp. 172-174, Jan. 15, 1984. This technique uses, for example, two input beams and a probe beam with a nonlinear medium selected so that the absorption of a single input pulse changes the refractive index enough to shift the Fabry-Perot transmission peak of the probe beam by approximately one full width at half maximum. Of course, the peak returns to its initial wavelength after the medium relaxes. However, the probe transmission after the input beams are incident on the etalon determines the output. Pulsed operation was also contemplated and even preferred. This type of logic element will be referred to as a dual beam device as the device distinguishes between two beams, in this case because they are at different wavelengths.
Similar work has described, for example, optical modulation by optical tuning of a Fabry-Perot cavity but the potential for performing logic operations was not explicitly described. The transmission of a single beam through the cavity was modulated by a control beam which varied the refractive index of the cavity medium thereby changing the refractive index for the signal beam. See, for example, Applied Physics Letters, 34, pp. 511-514, Apr. 15, 1979.
Although optical logic elements afford, at least theoretically, enormously enhanced switching capabilities as compared to electronic logic elements, it must be understood that they are also subject to the same fundmental physical limitations as are electronic logic elements. One such limitation is thermal in nature and caused by the necessity of removing the heat produced by absorption during logic element operation. The existence of this limitation was realized early in the development of optical logic elements. See, for example, Applied Optics, pp. 2549-2552, December 1969. Lack of adequate heat dissipation may alter the optical characteristics of the logic element, thereby either rendering it inoperative or degrading its operating characteristics. Additionally, an increased ability to dissipate the absorbed energy permits the packing density of the optical logic elements to be increased.
Accordingly, several approaches towards improving the thermal stability of optical logic elements have been taken. The most basic approach to thermal stability involves minimizing the absorbed power density. The InSb optical bistable devices reported show no apparent thermal instability. However, these devices had relatively large volumes and were thus not optimized for lowest power operation. The SEED device previously mentioned used optically induced electric fields, rather than an optical resonator, to modulate the beam and thus required relatively low optical power densities. However, its volume is also relatively large.
Another straightforward approach to thermal stability involves heatsinking. Of course, all devices are heatsunk to some extent. However, both the materials used in the device and its method of operation may limit the effectiveness of heatsinking. For example, dielectric layers having low thermal conductivity may be used between the nonlinear medium and the heatsink thereby reducing the effectiveness of the heatsink.
Another approach involves minimizing the change in dielectric constant caused by variations in the heat sink temperature. This is effective as the output characteristics of the device depend upon the dielectric constant. This approach might be realized by operating the device at cryogenic temperatures where the slope of the bandgap energy versus the temperature curve is a minimum for many semiconductor materials. Another approach increases the dielectric tolerance, that is, the maximum allowable change in the dielectric constant which does not significantly alter the operating characteristics. This approach requires a large nonlinearity which is generally available at low temperatures.
Yet another approach to thermal stability uses pulsed operation. This involves lowering the duty cycle to reduce average power absorption. However, it also generally reduces data throughput. To make the duty cycle an absolute minimum without reducing the data throughput, the optical signals must be pulses having a length which is short compared to the medium relaxation time. Logic operation then requires the medium to be excited only for the duration of the pulses rather than for the entire clock period. The pulse duration is then infinitesimally small from the point of view of the medium. To operate in this mode with positive gain, the device must distinguish between two beams.